Flammable deserts: understanding the impacts of fire on southwestern desert ecosystems of USA

摘要

Traditionally, fire has been seen as an important modeler of ecological systems, especially through the history of fire-adapted systems (e.g., savannas and Mediterranean ecosystems). Nevertheless, hot deserts in the southwestern US (the Sonoran and Mojave) are thought to be non-fire adapted systems due to the scarce woody plants and insufficient vegetation cover to carry fire. Over the last few decades, however, fire have become more prevalent within the mentioned deserts because of the recent invasions of exotics grasses that now sufficiently provide a continuous plant coverage that is able to spread fire through the landscape. To date, however, fire effects on desert ecosystems remain unclear. I this dissertation I examined the impact of fire in changing the spatial distribution of creosote bush (Larrea tridentata) in burned areas of the Sonoran Desert (Chapter 2). In Chapter 3, I assessed the effects of fire on changing the availability of soil nutrients, comparing burned and unburned areas of the Mojave Desert. In Chapter 4, I moved forward and investigated flammability characteristics of desert plant species, and how these properties were related to the spread of fire. Overall, I found that (i) fire changed the spatial pattern of L. tridentata shrubs that survived the fire, compared to areas not affected by fire; (ii) over the short term (i.e., 6 months after fire) there was an increment of soil nutrients (N and K) within burned areas, compared to unburned areas, but over the long-term (i.e., 7 years after fire) N and K decreased, especially under the canopy of L. tridentata shrubs; and (iii) native plant species exhibited flammability characteristics that confer them properties of \u22igniters\u22 of thicker fuels, whereas exotic invasive species presented flammable properties of \u22spreaders\u22 of fire. In summary, the interplay of the three major ideas described above provided insights about the potential novel plant-fire dynamics in the southwestern US deserts. Given the flammable characteristics of \u22spreaders\u22 and \u22igniters\u22, the prevalence of wildfires may be continued over time. While fire can kill large sections of L. tridentata shrubs, this phenomenon would provide more openness in the landscape to be colonized by exotic invasive species. Over the long-term, and as soil nutrients tend to decrease within burned areas, it would be difficult for native species, including L. tridentata shrub, to recover after fire. Thus, heterogeneous fertility islands once associated with Larrea shrubs may disappear and become replaced by more uniform nutrient landscapes, dominated by exotic invasive grasses.